A dune "slack" is the valley between the sand ridges. It is formed either by the erosion of dry sand down to the wet sand at the water table, as commonly happened at Newborough, or by the enclosure of low marsh (e.g. saltmarsh) by a succession of sand spits.

A very different vegetation, from that of dry dunes, develops in slacks, where the sand is within reach of the dune system's freshwater table. Under these conditions dune slacks develop, which are often characterised by a marked annual fluctuation in water level. Water levels in slacks normally reach a peak in early spring, when many are flooded for periods of several months. They then fall sharply through the summer, reaching up to 2 m below ground level before starting to rise again in autumn (Ranwell 1972a).